Digital techniques for the transmission and reception of sound information, sometimes referred to a digital audio broadcasting (DAB), have progressed over the past few years and are anticipated, on a worldwide basis, to replace the present frequency modulation (FM) method of transmitting audio and other information. Digital audio broadcasting (DAB) is not only anticipated to replace FM modulation, but the fidelity of audio signals transmitted and received by DAB systems will be greatly enhanced, making DAB's acceptance welcomed worldwide.
One such DAB technique, the Eureka-147 digital audio broadcasting system, has been accepted around the world as an excellent technical solution for digital sound broadcasting to the mobile environment. The Eureka-147 DAB standard ETS300401 specifies a digital transmission technique for satellite, terrestrial, and cable distribution of sound and data in accordance with the Eureka-147 format.
The Eureka-147 standard ETS300401 specifies a Coded-Orthogonal Frequency Division Multiplex (COFDM) modulation technique, wherein the digital information is transmitted in the complex frequency domain using .pi./4-shifted D-QPSK on each of multiple carriers with 2 bits per carrier. A transmitter of such a DAB signal is required by ETS300401 to use an Inverse Fast Fourier Transform (IFFT) to convert the signal from frequency to time prior to transmission thereof.
Referring now to FIGS. 2A-2D, the format of broadcast information, in accordance with the Eureka-147 ETS300401 standard, is shown. The digital information depicted in FIG. 2A is defined by frames of information, such as frame 68. Frame 68 defines a structure having a juxtaposition arrangement that includes a synchronization channel 70 which occurs first in time in frame 68, followed by a Fast Information Channel 72 (FIC), which is followed by sets of data symbols which are arranged as successive time multiplexed subchannels, sub-channel 1, sub-channel 2, sub-channel 3 . . . sub-channel N, shown respectively by blocks 80, 82, 84 . . . 86.
The synchronization channel 70, shown in FIG. 2B, comprises symbols designated null symbol 76 and time frequency phase reference symbol (TFPR) 78. FIC 72 includes a number of data symbols illustrated in FIG. 2B as data symbols 72.sub.1, 72.sub.2, . . . 72.sub.j. In accordance with the ETS300401 standard, the FIC 72 may include either three or eight such data symbols. The synchronization channel 70 is added at the beginning of each frame 68 to permit the receiver to synchronize, both in time and carrier frequency, with the stream of data symbols.
As seen in FIG. 2C, each of the sub-channels of data, such as sub-channel 1 (80), is further defined as containing multiple adjacent data symbols, data symbol 1, data symbol 2, data symbol 3, . . . data symbol p, respectively shown in blocks 80.sup.1, 80.sup.2, 80.sup.3, and 80.sup.P. With reference to FIG. 2D, each data symbol within a particular sub-channel, such as the kth data symbol of sub-channel 1 (80.sup.k), is further defined as containing multiple carriers, carrier 1, carrier 2, carrier 3, . . . carrier n, respectively shown in blocks 80.sup.k.sub.1, 80.sup.k.sub.2, and 80.sup.k.sub.n, wherein the n carriers are spread over a frequency bandwidth of interest. Each of the carriers are phase modulated between adjacent data symbols so that each carrier is phase modulated over time. Thus, each of the n carriers are transmitted simultaneously as a data symbol, and each of the p data symbols are transmitted at successive discrete time intervals.
Under standard ETS300401, the Eureka-147 system has the capability of operating in any of four operational modes, each including a different number of active carriers as set forth in Table 1 below. Since the transmitted information is encoded with two bits per carrier, the number of data points placed on each data symbol is equal to the number of carriers times two, thus defining the lengths of data vectors used to spread information across the frequency domain for each of the operational modes.
TABLE 1 ______________________________________ Eureka-147 Operational modes Mode 1 Mode 2 Mode 3 Mode 4 ______________________________________ Number of 1536 384 192 768 Active Carriers Number of Data 3072 768 384 1536 Vector Points ______________________________________
The .pi./4-Differential QPSK modulation technique conveys the information of interest in the rotation of each carrier's phase with respect to the previous symbol's corresponding carrier phase. The phase rotation applied to each carrier is determined by the next two bits to be placed onto that carrier. The data vectors applied to the data symbols 66 are further pre-scrambled before being applied to the carriers, wherein this process of pre-scrambling is known in the art as frequency interleaving. Finally, the ETS300401 standard allows for transmission of data at different rates so that the sub-channels 80-86 are time-multiplexed and are previously pre-scrambled in time, wherein this process of pre-scrambling is known in the art as time interleaving.
A DAB receiver must be capable of receiving the transmitted signals described hereinabove, synchronize with such signals in both time and carrier frequency, and decode the signals for replication of the original information. Decoding of the information in a Eureka-147 based DAB receiver requires the capability of performing a complex Fast Fourier Transform (FFT) to reverse the effect of the IFFT applied by the Eureka-147 DAB transmitter. Due to the four operational modes of the Eureka-147 system as set forth in Table 1 above, a Eureka-147 based DAB receiver must correspondingly be capable of performing various length FFTs as set forth in Table 2 below. It should be understood that, under ETS300401, Table 1 sets forth only the number of active carriers per mode, and that such active carriers make up only 3/4 of the total number of carriers for each mode. Table 2 thus sets forth the FFT length requirements for decoding the total number of carriers, N, for each mode of operation of the Eureka-147 system. It can thus be seen from Table 2 that a Eureka-147 based DAB receiver must have the capability to perform complex Fast Fourier Transforms of lengths 2048, 1024, 512 and 256 for each of modes 1, 4, 2 and 3 respectively.
TABLE 2 ______________________________________ Potential Data Active Vector Carrier Number of Data Operations Complex N Points Multiplier Vector Points 1 Mode ______________________________________ 2048 4096 3/4 3072 1 1024 2048 3/4 1536 4 512 1028 3/4 768 2 256 512 3/4 384 3 ______________________________________
Eureka-147 based DAB information is time and frequency interleaved prior to broadcast thereof in accordance with a complex IFFT process. In channel decoding and frequency de-interleaving (descrambling) such signals, known Eureka-147 based receiver systems utilize an arrangement of N-capacity inputs and output data storage buffers operable to store the N-carrier information prior to and following the FFT process. Within the FFT process itself, such known prior art systems further require two more N-capacity memory units for carrying out the FFT process and a fifth N-capacity memory unit for holding previous symbol values for the channel decoding process. Known prior art system thus require a total of 5N storage locations for decoding N-carrier Eureka-147 based DAB information.
Such prior art Eureka-147 based DAB receiving systems are expensive in terms of memory usage required for management thereof. What is therefore needed is an efficient technique for channel decoding and de-interleaving (descrambling) broadcast Eureka-147 based DAB information which minimizes memory required therefore.